Oh I'm so with you on this one. I still miss that feature, years later. The excuse of it not being compatible with audio tracks is, frankly, BS, because I'm perfectly capable of altering the tempo in my project by hand, then changing it back, so why can't that be automated? That is my "workaround" btw, if you could call it that. Another workaround I frequently use is this, which I know also completely sucks:
1. Say you're recording a flute track. Choose a single existing comping track that you can play back during recording, such as a piano
2. Solo the (empty) flute and piano
3. Select the clip in the piano over which you'll be recording
4. Edit the length of that clip to 200%, which makes it half as fast. Be sure to check both the Start Times and Durations boxes
5. Record your flute at half speed over the half-speed piano
6. Select both the piano clip you altered in step 4 and the newly recorded flute clip
7. Edit the length of both clips to 50%, restoring it to its correct tempo
8. Un-solo