On Fri, Oct 22, 2021 at 02:05:25PM +0200, IB Development Team wrote:

Man pages

https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-send
https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-receive

do not say anything about kernel versions required on sending/receiving 
side.

Is possible to do btrfs send/receive between systems with different 
kernel versions, i.e. sender with newer linux kernel 5.10 (btrfs-progs 
v5.10.1, debian 11) and receiver with older linux kernel 4.19 
(btrfs-progs v4.20.1, debian 10)?

The compatibility level hasn't changed for a long time, so in general it
should be fine to use older kernels and progs.
There were some small updates eg. fixing chmod and capabilities, with an
intermediate workaround, so this could be a (fixable) problem.

Does "btrfs receive" protect against applying incremental snapshot diff 
in case it's not compatible with btrfs FS on receiver or admin must take 
care of compatibility checks "manually"?

I'm not aware of any such checks that would be needed. I think there
were some problems with SElinux enabled on the receiving side, so I'd
expect this sort of problems and not inside the send/receive protocol
itself.

An update to the send protocol is underway so the question of
compatibility will become important.