Hello,
In March 2021, the support team stopped manually relegating teams who hadn't been affected by an issue with the promotion/relegation system.
However, you can relegate youself by joining a league which is about to finish their season and do purposely finish badly in order to be in the relegation zone. In Pro, you are likely to level up faster (as you'll be racing with higher level managers) so relegating to Rookie may not be a good idea in that respect.
More information on the removal of manual promotions/relegations can be found here:
https://igpmanager.com/forum-thread/42375