Since when can we Update iOS in Recovery Mode WITHOUT erasing data?

It may be able to update ios without losing data in recovery mode.

Whether it can or not will depend on exactly what the problem is. There is really no way to know ahead of time.

I have rarely ever seen it work. In almost every case, it’s necessary to restore, which wipes all data from the device.

Yes, it’s been that way for a long time. No it is not giving ‘false hope’ to users.

Does it really matter? If it works, great. If not, you're no worse off than you were.

Note: don't leave backing up your data until something goes wrong. You're phone can easily be set up to automatically back up every night. That way, you'll never lose more than 24 hours worth of data.