What chess openings theory from the early 20th century would still be solid at the club level today?

Out of all of the opening theory discussed in the writings of the early 20th century masters -- like the beginner books by Tarrasch and Capablanca -- which openings were well enough developed back then that the older analysis is still fine for modern club players?

(If any.)

Just about all of it.

Huh...That's very interesting to learn. A lot of threads and articles I've seen online about using the older books (like Tarrasch's) suggest skipping the opening sections because they're outdated.

Authors try to sell books.

If Alekhine or Capablanca couldn't bust a particular old line, why would you expect your 600-rated opponent to be able to do so?

At the club level which I assume you mean below Master? Anything is playable.

Any of the main lines played from 1900 to 1940 are fine for club level chess. In fact, I have often recommended going back and playing through the games of Capablanca, Alekhine, Lasker, and the other giants of the era. The annotators of the day did a good job of explain the lines, so today’s students may actually enhance their understanding of the openings. And you may find sub variations that have gone out of fashion and surprise your opponents.