As we already see many template providers offering single downloads with no/limited updates then this will be a problem.

Example.
I buy a template from provider x who marks all of their overrides as checked (why wouldnt they)
I no longer have support from the template provider so I dont get any more updates
Joomla makes updates and you as the site owner will never know about it because your template providert marked it all as ok

How do we resolve that?

You mark them checked for Version 5.0.2 for example. Its up to the template provider to offer the service or not. Also Template Providers have the bonus that they can say, that the template NEEDs to be updated.

I am just thinking out aloud

I buy a template from provider x who marks all of their overrides as checked for 5.0.2
I make some changes to those overrides
Joomla releases 5.0.3
Joomla tells me I have template overrides to check
Template developer issues update marked as ok for 5.03
I install that update and lose all my changes

If you update your template yourself, you always lose your changes. That has nothing to to with that issue.
Actually my problem here is that as a customer from a provider I have not clue what to adjust in their code.
Maybe the flag should be lost when a user edits the override. Thats true. Because then the guarantee by the template provider can't be held anymore.

real world example

Every template created for Joomla 4 that has overrides for com_content or com_contact needs to be updated for Joomla 5. So for a template provider to mark their template overrides as checked they will need to maintain at least two versions of every template (while joomla 4 is still supported)

Ok so the overrides are not correct for Joomla 5 anyway - then this needs to be adressed anyway or not ?

real world example

I just checked a leading template provider and they only offer one download for their templates and it is only fully compatible with joomla 5 but there is no mention of it not supporting joomla 4

I checked another leading template provider and they only offer one download for their templates and it is only fully compatible with joomla 4 etc

Yes I understand you, but then there is a general issue or not?
If you get the mention to check 44 Overrides from a template provider, do you contact them, or what do you do in that case?

Yes I understand you, but then there is a general issue or not?

yes there is a general issue but I dont see it as resolved by this proposal.

The first part of the issue is that Joomla did not communicate the major changes in template layouts between j4 and j5. If it had done that then at least part of your issue is easily resolved. hint search for all instances of itemprop and remove them

The second part is that I don't trust template providers to be 100% truthful about their templates compatibility. Based on a 2 second check of two leading providers they aren't now so why should they be in the future.

If you get the mention to check 44 Overrides from a template provider, do you contact them, or what do you do in that case?

Personaly I would check them

Finaly one reason that there are often such a large number to check is that when creating an override using joomla it will create an override for every file even if you dont change it. Personaly I think it is good practice after creating overrides to use the difference function and delete all the unchanged files. That massively reduces the number os files to check.

For example on one of my sites I have an override to change blog links to a ul from an ol BUT by default that will produce 8 files to check in the future.

@brianteeman your steps are wrong. You as an admin SHOULD NEVER touch the overrides that comes with any template, that was the whole reason for the child templates!

So the step

I make some changes to those overrides

is wrong and if you override template owned overrides then you have to do it ONLY through a child template. Also that shouldn't be an option but rather imposed if the project really wants to have things that are consistent and predictable.

That said a template could come with it's own overrides (that admins should not tweak) but also an admin could create any override for a layout/ext template. ATM there's no distinction in the CMS that these 2 types are different and this is exactly what Elisa is asking here: ie distinguish the template owned overrides from the ones that are user/admin owned.

FWIW the PR prior to what became the child template did exactly that ref

Related/duplicate #38653?