Gen I exclusive track doesn't equal fanservice. Callbacks to Gen I is fan service.
Sorry, I'm a little confused over your response. Let me make sure I understand, are you calling the Hall of Fame theme a callback to Gen I, or is it something else you're talking about?
If you are referring to the Hall of Fame theme, I'd argue it's no more Gen I fan service than Pokémon Centers or Gyms. They're things that exist across the entire series, and using them doesn't mean they're forcing fan service into the game. My assumption is that they used the Hall of Fame theme because they saw it as a theme of the Pokémon League, which it almost is, in a sense.
Speaking of the Hall of Fame, I was disappointed that this game lacked it. The title screen featuring you standing by your team is a cool replacement, but in the moment of victory, I really missed the celebratory aspect of the Hall of Fame. Honestly, I find it surprising that of all games, this would be the one not to use it, given Galar's extreme focus on the league, I would think the Hall of Fame would be an important aspect to memorialize their champions. Not to mention a Hall of Fame works well with the sports-like theme of the Galar League.
I also find it funny that the team pose on the title screen answers one of the first questions that came to my mind during the reveal trailer, which was "Why does the main theme use the Hall of Fame theme?"