Does it mean some changes in adding those via TV Everywhere support?
TV Everywhere access is based on the contract that your provider (YTTV this case) has with the channels. Some only have a streaming agreement through their app, and some allow other ways to access it. This the list for TV Everywhere for YTTV: https://support.google.com/youtubetv/answer/7556177?hl=en#zippy=%2Ccurrently-available-tv-everywhere-sites
You can see for yourself whether sites accept your YTTV login by going to the site yourself and seeing what providers it accepts.
I know how the TV Everywhere works. You redirected me to old support post, while YoutubeTV added 6 channels essentially just today for everyone.
@beyo77 I think we are missing some context here. Here is an article detailing what changed:
https://9to5google.com/2021/03/16/youtube-tv-new-viacom/
I would assume if YTTV has TVE access to these channels and they have a live stream on the web they could be accessible via Channels.
Fastest way to find out whether the new channels are available via TVE is to perform a rescan. Go for it. 
Check for yourself by going to any of the Turner networks and seeing if YouTube TV is added to the provider list, that is going to be more accurate than any support doc or unofficial list. I tried TNT's stream and it doesn't look like YouTube TV is there.
Also, it's possible it may never be added. TNT, CNN, and other Turner networks have always been available to AT&T TV (formerly DirecTV Now) customers through their app and they never got TVE access.
I will but my understanding was, that if we know that Turner channels are not supported on TVE for Youtubetv they will not be included in scan, because developers programmed that to optimize codebase.
Your understanding is incorrect.
For what it's worth, I ran a rescan last night and the new Viacom channels (NickJr and DABL, for example) were identified by the scan even though they ultimately weren't added to my list of channels since they don't support TVE.
I still think that the best way to see what's accessible and what's not is to simply visit the webpage for the channel you're interested in and see if you can log in with your YTTV credentials to get a live stream. That seems to be largely what a "rescan" does, as evidenced by the repeated "somebody tried to log into your YTTV account!" messages that I get during a rescan as it laboriously checks each channel. Perhaps there's yet another filter on top of that for TVE, but I suspect that it starts with whether the provider accepts your YTTV credentials at all.
Good to know, Looks my approach is different
Apparently there is no support for MTV2 and MTV Classics because for reason unknown there is only MTV live stream on their page present 
I think DABL was available before no matter provider.
That's not big deal, I can always watch "Paw Patrol" in the YTTV app 
MTV2, MTV Classic, Nickelodeon and Nicktoons not available via TVE for live streaming. Only Nick Jr. has a live stream