I bit the bullet and restored back to Mojave.
For anyone who has the same question - the answer is yes, I can still see my backups on the Time Machine hard drive, however the Catalina Time Machine format is different to the Mojave one, so I can't access files from the newer backups using the Time Machine app.
However the data is there and can be restored using something like
Edit with another lesson
Something I didn't know but accidentally saved myself from: Photos on Catalina (Photos 5) stores your photos differently to Photos on Mojave (Photos 4).
Photos 4 uses a logical table structure like:
Photos 5 uses a uuid structure:
For some reason, Time Machine doesn't backup the Photos database. So my method of downgrading and then rsync'ing the newer backup wouldn't work for Photos since rsync would not recognise the different folder structures. Luckily I was paranoid and took a separate, manual backup to a different disk before downgrading.
This manual backup had everything, including the Photos database. This meant I was able to use the python package osxphotos to read the Photos 5 database and pull out any photos newer than my old backup, then import those into my restored Photos 4 library.
osxphotos --db backup/Photos\ Library.photoslibrary export --export-by-date --from-date 2019-07-01 restored-photos
tl;dr before downgrading, take a separate, manual backup of your Photos library, or at least the
./Pictures/Photos Library.photoslibrary/database folder so that you can restore any photos newer than your Mojave backup.