The documentation for CYCLE claims that RESTRICT is required in stadard SQL.

That's wrong. ISO/IEC 9075:2016-2 defines CYCLE as follows:

<cycle clause> ::=

CYCLE <cycle column list> SET <cycle mark column> TO <cycle mark value>

DEFAULT <non-cycle mark value> USING <path column>

(I havent seen RESTRICT in elder versions either).

I think this happend with MDEV-20632 on 2020-02-28 15:21 when RESTRICT was added to the MariaDB implemenation syntax, but accedenally also added to the standard SQL example.