@Atreeon I will try to clarify some of the Free Plan features and answer as many of your questions as I can.

Before I try to answer all of your questions, I will break down the Free Plan roles and types of access to a board in the Free Plan and moving forward I will simply refer to these and Team and Public. You can find these settings in a board’s Share settings from either inside of the board or by using the ellipsis “three-dots”menu:

First, let’s talk about the options available at the Team level:

1. Can view - Any member the Free Plan team (either Team Admin or Member - the only two roles) can view the board in read-only mode - the will not be able to edit it - only the board owner (the person who created the board) will still be able to edit the board.
2. Can comment - Now any team member can view the board, add new comments to objects that did not already have comments, as well as add additional comments to existing comments.
3. Can edit -  This is the default setting of newly created boards in a Free Plan.
4. No access - aka, Private Boards ← as this Miro Help Center article states, and as does the help bubble in the above screenshot, private boards are no allowed in the Free Plan.

Public board access:

NOTE: “Anyone” really means anyone - they don’t even need to have a Miro account:

1. Can view - Again, anyone with the link can only view the board.
2. Can comment - Similar to the Team-level setting, anyone with the link can create & add new comments.
3. Can edit - This option is not available for Free Plans. Only team members can edit the board. Public edit access to a Miro board is for paid plans only. This feature is commonly referred to as Guest Editor access - check out this article → Collaboration with Anonymous Guest Editors. It is also worth noting that paid plans can also password protect public boards.
4. No access - This is the default setting and as it sounds means there is no public access allowed to the board.

Now that we’ve gone through that, some of your questions may have already been answered, but I will address them in summary.

On the free plan I understand the boards are open for anyone to view

When boards are first created in the Free Plan, they are editable by anyone on the team. And, no matter which Team-level setting you choose, boards on the Free Plan will always be at least viewable to all team members - there are no private boards in the Free Plan. Boards are also not open for “anyone”, i.e., they are not viewable by the public.

 How discoverable are they?

All team members will always see all boards on the team’s dashboard in Miro.

Can anyone with the link and a username edit them?

No. Anyone on the team can edit them only if the board owner has set the Team-level access to Can edit. Non-team members will never be able to edit the boards.

Can you see who has viewed the board?

Not on the Free Plan. Only the Enterprise Plan plan offers Audit Logs.

Are the URLs easy for anyone to discover?

No. Miro takes the appropriate steps to stop search engines from indexing Miro board URLs. However, this obviously does not prevent someone from posting a Miro board URL on the Internet. Also, I suppose someone could write a malicious program to randomly generate Miro board URLs, but that’s what board password protection  is for. Note that, if you decide to sign up for a paid plan and want to use Password Protection, it may not show up on your account right away as per the following message on the password protection help article:

To be notified when this feature is available for all paid plan accounts, I suggest that you visit the News & Announcements section and click on the Subscribe button.

If you are on a paid plan and then revert back to a free plan do your once private boards become visible to anyone?

No, nor will you be able to see the boards in your Free Plan. One poor choice of words that Miro makes (in my opinion) is the board-level option (using the ellipsis menu from the dashboard) called Move to account:

I say account is the wrong word given the following:

1. A Miro account is created with an email address which makes the user a unique team member.
2. This team member can now be added to a virtually infinite number of teams as a team member.
3. For every team an account holder is a member of, that team shows up in the Miro dashboard’s left-vertical menu:

In the above example, my account using my *********@gmail.com email access is a team member on six Miro teams - two Free Plan, one Education Plan, and two Consultant Plans.

That being said, I believe the option should read: Move to team (but someone, please correct me if I am missing or not considering something).

Back to your question after I went off on a rant:

If you are on a paid plan and then revert back to a free plan do your once private boards become visible to anyone?

If you had a paid plan and were to have either created a board in your

1. paid plan; or
2. Free Plan and then used the Move to account option to move it to your paid plan

once you stop paying for the paid plan, you will no longer be able to access the board. I believe that you will see the paid plan team you created from your Miro dashboard, but when you try to access it you will be directed to upgrade/pay. However, the boards will be there waiting for you.

Let me know if I missed anything or if you have any more questions!