(filed under Storage Engine - InnoDB because there is no component "Storage Engine - Blackhole", but it affects InnoDB compatibility)
Previous versions of InnoDB had a max index length of <1KB. Most modern versions of MariaDB increase that to 3KB+ (the innodb_large_prefix option).
Blackhole, while it is not an engine enabled by default on MariaDB, it is still shipped with it, and may have some limited usage for which it would be useful, even if can create many issues. One of the usages is to replicate to a separate server but ignore all data. However, there is a big compatibility issue between InnoDB and blackhole, and that is that indexes on Blackhole cannot be larger than 1KB.
max index length for Blackwhole is an arbitrary constant (https://github.com/MariaDB/server/blob/10.6/storage/blackhole/ha_blackhole.h#L70 ), and increasing it is very simple (no complex data to manage), but Oracle MySQL ignored the upstream bug report: https://bugs.mysql.com/bug.php?id=53588
Percona has applied such a patch: https://github.com/percona/percona-server/pull/2000 , and it has already a 3KB limit. I don't see a reason not to apply the same patch (or a similar, it is a one line change + tests) as it would improve compatibility, not break it.