I travelled from the Belgian border to the Swiss border over several days (NRW-RP-Saarland-RP-Hesse-BW, in a roundabout manner). My trains were not any more crowded than I would have expected them to be in June 2019.
In contrast to Albert Beale, my ticket was checked on almost every train, including at 11pm and at 5am (actually twice on the same train within 30 minutes by different people, and none of the other continuing passengers made a fuss!). Nobody has asked for ID yet. Also I guess I'm not travelling on tourist-heavy routes, but it seems that very few passengers are using 9 euro tickets, most of them are still presenting some sort of season ticket card.
I also took some buses and none of the drivers were bothered about tickets at all, they waved me on when I tried to get it out of my pocket.
In previous years I travelled around similar areas with day tickets from BW, RP, VRR, VRS, etc., and those were very rarely checked on trains, yet bus drivers always wanted to see them and would get confused when presented with tickets bought at DB machines (except in some cities where it seemed to be the norm to board by back doors - mainly Wiesbaden IIRC).
I also took some ICEs in Germany where the regional timetable was poor - and my (separately purchased) tickets were not checked on any of these.
In Switzerland I used Saver Day Passes. My ticket was rarely checked on board trains including some very long journeys, even though I would have expected it to be checked quite frequently based on my 2019 and 2020 experiences where they also asked for ID often.