Who was the first modern grandmaster? According to Wikipedia, the modern system for awarding FIDE titles evolved from the &quot;Dorazil&quot; proposals, presented to the 1970 Siegen Chess Olympiad FIDE Congress. On the same Wikipedia page it is written that, to qualify for the grandmaster title, a player needed to achieve three GM norms within a rolling period of three years. Exceptionally, if a player's contributory games total thirty or more, then the title could be awarded on the basis of two such norms. (I guess the 2500 rating requirement was added later.)
I would be curious to know who was the first player to obtain the GM title under this new system. A bunch of players obtained the GM title at the beginning of the seventies (see here).
The Wikipedia reference gives 3 names for 1970 - Bukhuti Gurgenidze, Walter Browne and Anatoly Karpov.
Wikipedia's Bukhuti Gurgenidze article lists only the Tbilisi tournament taking place in December 1969.
Walter Browne's Wikipedia article says this:

His zonal result earned him an invitation to an international
grandmaster tournament in San Juan, Puerto Rico. There, he gained the
Grandmaster title by tying for 2nd–4th places, with Bruno Parma and
Arthur Bisguier, behind reigning world champion Boris Spassky

So, San Juan was his final norm.
In neither case do we know from these sources when the actual title was awarded because titles are not awarded until federations apply for them on behalf of the players and applications cannot be considered and approved until there is a FIDE congress or board meeting.
For Anatoly Karpov we do have a specific date in Bill Wall's blog:

In June, 1970 Anatoly Karpov tied for 4th place at an international
tournament in Caracas, Venezuela, gaining a Grandmaster norm.  He was
awarded the Grandmaster title at the 1970 FIDE Congress in Siegen in
September, 1970 at the age of 19. He was the world's youngest
Grandmaster.

Given that Browne and Gurgenidze's tournaments took place in 1969 it is unlikely that their title applications came in after Karpov's. The Siegen congress was the congress at which the changes were made and so it could be that all three were agreed at the same time. In any case, Karpov is certainly one of the first.