Quoting from developer.kde.org:
Key required in older versions of KDE (deprecated):
As explained above, in KDE Frameworks older than version 5.85,
the servicemenu files are in directories kservices5 and
kservices5/ServiceMenus in the generic data locations.
In this case, the following key is required in addition to Type,
MimeType and Actions.
ServiceTypes=KonqPopupMenu/Plugin
I understand (wrongly?) that ServiceTypes is deprecated in KDE6.
I’m under this impression too, although file location has changed.
Anyway, without X-KDE-ServiceTypes, my custom service menu is restored.
Thank you for your replies.