It seems that the dependencies bundle depends on a very old version of javax.mail, which depends on a very old version of javax.activation, which is not installed automatically.
Manually adding javax.activation 1.1.0 (e.g. from Eclipse 2019-09) to my Eclipse installation resolves the problem.
When installing the feature group in 2020-12, everything seems to work, but the plugins cannot start. The Eclipse error log shows:
It seems that the dependencies bundle depends on a very old version of javax.mail, which depends on a very old version of javax.activation, which is not installed automatically.
Manually adding javax.activation 1.1.0 (e.g. from Eclipse 2019-09) to my Eclipse installation resolves the problem.