Can I upgrade MariaDB replication infrastructure gradually? I have 2 masters and 1 slave (replicating form both masters).
M1- MariaDB 10.4.11
M2- MariaDB 10.4.16
S1- MariaDB 10.4.12
I want to upgrade this LIVE system to 10.5 with possibly 0 downtime. The idea is this:

upgrade slave S1 (this is used mainly for backups, not queried at all)
switch all traffic to M2, stop M1 and upgrade M1
turn on M1 and sync it from M2 - at this stage I will have active master M2 running old 10.4 MariaDB, but both standby master M1 and slave S1 are already on 10.5
switch all traffic to M1, stop M2 and upgrade M2
sync M2 from M1 and keep it as standby master

I guess my main question is - Is MariaDB 10.5 &lt;-&gt; 10.4 replication compatible? Any particular issues I should look for?
Thanks!
A MariaDB replica can replicate from an older master. The other way around won't work.
You should add a step 2.5: stop replicating from M1 to M2 (even if it's just a standby master). After step 4, M1 can replicate from M2 again.