Federation:
Various Classes are still used, though usually in small number. By the Dominion War,
a bunch of old vessels, including FASA vessels, were relaunched to fight the Dominion, composed of:
load of Excelsiors and Mirandas (obviously)
4 Chandleys
2 Loknars
4 Bakers
9 Andors
1 Babcock
1 Lenthal
1 Thufir
6 Ranger IIs
3 Keiths
Many of these vessels were shredded, despite occasional upgrades, however
Federation needed as many vessels as it could get up and running.t

Klingon Empire
The Klingons had a tendency to constantly rely on older vessel classes for
easily over a century, not just basic design schemes, such as the Bird of Prey template, out of a strong value for reliable and well-proven ships .
They pushed many of their older classes to minor naval duties by the Dominion
War, however these vessels still fought astonishingly well compared to older Federation Classes in use during the war, with relatively few losses.

499 K't'ingas
23 D-10s
64 K-22s
26 D-2Bs
8 D-2As
89 L-9s
34 L-42s
4 D-11s
3 K-15s
5 K-17s
5 K-27s
4 L-24Bs
200 K-32s

Romulans
The Romulans retired all of their 23rd century designs by the 2330s, entirely
modernizing the fleet. None of their older ships have ever been spotted during the Dominion War.

Gorn
The Gorn used a handful of BH-2 Battleships and MD-8 Light Cruisers even into
the 2390s, but were quite willing to rely heavily on newer designs.

Orions

The Orions used Wanderers constantly for all of the 24th Century, finding theh
old Blockade Runners to still be useful after a century of use. New vessel designs
were used, but the Wanderer always remained prominent

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