Is it necessary to buy BF6 if the PUBG mode in BF6 is free?

Hi steames01​ 
You are probably referring to Battle Royale mode.
Unfortunately, no information has been released at this time regarding the availability of Battle Royale mode in BF6 (free, separate, or paid...)
I therefore invite you to wait for an official announcement from EA.

Here is the only official information available : https://www.ea.com/games/battlefield/battlefield-6/news/community-update-battle-royale-testing 
Have great day 😉