I agree we're doing two different things here, but wonder what the solution is for this. Would we create another flag like jQuery.disableDeprecationWarnings or something like that?

Yeah, naming is hard. ;) Would a single API be enough, considering there are use cases both to only enable patches required to make the current major version of jQuery work post-upgrade and to only enable deprecation warnings for a future major?

We could also bake it into jQuery.migrateDisablePatches & jQuery.migrateEnablePatches as some special keys like compatibility-patches & deprecation-patches (again, better naming suggestions welcome). Together with that, we could introduce a special key all that would disable all patches in case someone wants to enable one by one instead of disabling one by one.

Then we'd just need to keep a list of compat patches and compare those special keys against that. Thoughts?

I've been thinking about this more. From my original list, the first point - about differentiating between warnings for APIs that still exist and patches - seem less important to me than the second one - about not allowing patches restoring behavior from earlier jQuery versions than the one from which the upgrade process starts. Let's focus on that.

This is important as Migrate will now support both jQuery 3.x & 4.x and we don't want people to restore the 1.9 behavior when trying to upgrade from 3.x to 4.x.

I think we need to be able to specify the jQuery major from which the update starts, considering that breaking changes should generally only land in major releases now (jQuery 3.5 was a one-off BC due to a security issue). An API like:

jQuery.migrateUpgradingFromJQuery( major );

where major can be 1, 2, 3 or 4. If set to 1 or 2, all patches would be enabled. If set to 3, only the ones covering APIs removed in 4.0.0 or later. If set to 4, only the APIs that still exist would be patched to provide warnings.

I'd also propose for jQuery Migrate 3.x to have it set to 1 by default, and for jQuery Migrate 4.x - to have it set to 3, so that the default patching behavior is restoring less legacy cruft.

We also need to think how to implement that - we need to somehow associate each warning with a jQuery version where a related breaking change occurred - or none, if the warning is for an API that still exists in all stable jQuery versions. It doesn't seem clean to repeat the major in each migrateWarn call, so we should have a way to register this metadata in some kind of a central registry. I don't want to keep it in a single place, though, so that we don't end up with legacy registrations for warnings already removed. Perhaps we need an internal API like registerWarningForMajor( major ) where major can be 3, 4 or undefined - the last one for APIs that still exist. We should also incorporate a check into migrateWarn that a registration was already done, or throw an error.

If someone has a better idea for how to implement this, or what a good API would look like here, I'm all ears!

On a related note, I'm also considering making Migrate 4.x only support running with jQuery 3.5.0 or newer. jQuery 3.5.0 was released 4.5 years ago, it has no reported security issues and is a breaking change release despite being a minor update. It'd also make our tests quicker due to a smaller set of tested jQuery versions.

Discussed in meetings and landed on the following:

• migrate 4.x and future versions will differentiate between warning about deprecations and removals as normal, but treat upcoming changes as info and not warnings. 3.x will remain special since that's how it's worked for a long time.
• keep migrate versions in line with core versions. simplify 4.x.

This is handled in a way Timmy described above for 4.x in #555.