This was discussed a ton in debian-devel. First, the tmpfs doesn't take much space already, and /tmp became a folder where persistence should not be expected over the years.

The problem with /tmp was many people and apps used it as an inter-user communication medium and expected persistency there, so it created both security problems and wasted disk space over time.

Since not many packaged apps used the /tmp like that and used the folder the way it should be used, the change was made.

I'm running Debian testing on one of my systems, and the change created no ill effects whatsoever. Not eating SSD write cycles can be considered a plus, even.

However, as I also noted in the relevant thread, the approach might have a couple of downsides in some scenarios.

If you have the time and the desire, discussion starts at https://lists.debian.org/debian-devel/2024/05/msg00014.html