if you want to use tor for certain applications you can also use proxychains and tor[commandline] together
sudo apt-get install tor proxychains -y
proxychains curl -s https://ifconfig.me
[for checking public ip address of sender]
u can proxify any networking command line application by preceeding proxyhains before it
If the ISP can reveal that the IP address/port pair you're talking to is a Tor bridge, then yes, that means they can determine that when you send traffic to that IP address/port pair that you are using Tor.
Perhaps, if you are subject to a regime that considers private communication to be, ipso facto, evidence of something unsavoury, then yes, using Tor ...
Only the developers of TBB could answer your question with 100% accuracy. A couple of possible reasons include:
The TBB developers haven't gotten around to it yet -- either flipping the switch, or (probably more likely) doing an analysis of the privacy trade-offs that would result from enabling such a setting; or
The TBB developers are dead-set against ...
I recommend looking into DNS over HTTPS for any clearnet traffic that you may be touching, including if you are running your own exit node: Here is an example along with instructions on running your own.
I am running my own at https://jcdns.fun to experiment with it and have both my Firefox Nightly browser and Chromium browsers pointed at it for all DNS ...