Looks like Maidstone are the first team below level 5 of the pyramid to make it to the 5th round since the 1925-26 season?.
I think when before 1979 when the National League started, the Northern League and Southern League was level 5 and that seen Blyth Spartans, Yeovil and Colchester make the 5th round.
Then in 1985, Telford became the first non-league team to get at least that far after the introduction of the Alliance Premier League, today known as the National League and sat on the 5th tier.
After this we had Kidderminster Harriers (1994), Crawley (2011), Luton (2013), Lincoln* and Sutton (2017), then Boreham Wood (2022) all appear in the 5th round as teams from that same league, regardless of it's name changes (Football Conference and then National League) all as 5th tier sides.
So, today marks a bit of FA Cup history.... Maidstone United are the first ever team since 1925-26 season from below level 5 of the football pyramid and the first below level 1 of the non-league system to appear in the 5th round of the FA Cup.
Before 1925 I think the highest level of non-league football began at like level 3?**, so it might be an FA cup first for Maidstone and not just after 1925-26?.
* Only non-league side to make the Quarter Finals, although they had to play in the 5th round at first to get there.
** Spurs won the cup in 1901 as an Alliance League side, which sat on level 3.