Tempe
The standpoint “when everyone worked in reels” is incorrect. We STILL work in reels. This is accepted format for feature delivery, fiction or documentary.
Of course your post house is free to request whatever format they want, but CMX 3600 is accepted standard and yes, 999 events is the limit. It’s a holdover from the analog tape days and isn’t going to change. Editors have asked this question for years.
It’s generally accepted to deliver each “A/B” reel (two ten min reels joined together separately), picture and audio. Then the audio engineer uses Pro Tools (the standard) or similar software to do any additional sound editing, add ambience, sound fx (in camera or Foley), ADR, Music, etc.
If they can’t accept your CMX list, maybe you can make an OMF from your edit software and they use that?