require_protection: Any way to make this extensible? Basically to easily support any protection mod instead of separate implementation for every protection mod?

It should be possible by adding last override for minetest.is_protected and make 2 calls from there: actual player name and empty string.

• if both return false then there's probably no protection.
• if first returns false and second returns true then it is protected area where player has access.
• if both returns true then it is area protected by another player.

Not sure if that would work well enough and what corner cases there would be (for example open areas) but would be nice to have generic solution that could work with any protection mod and to require minimum changes to actual protection mods.

@S-S-X Your idea is good, but how can any protection mod know whether the position is protected and accessible for the player or whether it's unprotected?
Some protection mods also have options to allow access to everyone. The area would be "protected" but impossible to distinguish from open maps.

but how can any protection mod know whether the position is protected and accessible for the player or whether it's unprotected?

Protection mods usually do know this for their own protections, suggestion with dual call was exactly to make call that likely returns true for every protection to check for any protection from any protection mod and then another call to check for actual protection against player.
Access for everyone (open areas) and such however probably makes this way too complicated approach.

Some protection mods also have options to allow access to everyone. The area would be "protected" but impossible to distinguish from open maps.

Yeah, that's exactly one of those corner cases I've mentioned... and probably dual calls to is_protected wont work well enough as it would break many open/public areas.

One way I could think of that might possibly work would be to return both "is_protected" and protection state from minetest.is_protected (should be fully backwards compatible and not needed at all for mods that do not do "allow all" ever):

function minetest.is_protected(...)
    -- access granted but position is protected
    return false, true
end

That would require some changes to protection mods but would be minimal just adding second return value.
Mods that do not implement both return values (second is nil) can be handled with default until said protection mod adds support (it would be problem only with ones that add "public" protections).
Grab and forward return values from chain with mods that do not return protection status can be done with __index meta method (metatables can be extended so multiple can be kind of stacked).

No future in this PR.