It was there when your account was banned, yes.

Also this:

Can I share my phone number between multiple accounts?
Yes. You may use the same phone number on multiple accounts. Accounts with the same phone number may be treated as the same identity with respect to the Steam Subscriber Agreement, policies, or restrictions.

https://steamproxy.net/steamhelp/en/faqs/view/7EFD-3CAE-64D3-1C31

i just asked was it there 5/6 years ago i didnt get banned yesterday lmao but again i dont have any problem with that part, all i want is banning phone numbers forever or atleast for a few years

It's there. When you link a number to any account, it tells you what will happen if cheating occurs.

It also says this, that you didn't read:

How do VAC bans relate to phone numbers?
VAC bans are applied to all accounts sharing a phone number at the time of the infraction. For this (and many other) reasons we do not recommend sharing your phone number or using another user's phone number on your Steam account.

i dont know if that notification you talk about was there 5/6 years ago but i have no problem with that part at all i am curious is that why they let that phone number for further use. they should just block it cuz currently cheaters can easily rebuy the game via different account and connect their phone number without anyproblem

You are warned when linking accounts by a phone number as to what would happen to all accounts linked to that number, if cheating occurs. You are warned of these consequences EVERY time a phone number is linked to ANY account. Your fault for not reading those warnings.

https://steamproxy.net/steamhelp/en/faqs/view/647C-5CC1-7EA9-3C29#phone