Tanking trinkets in BFA suck for survivability.
That’s fine, you probably shouldn’t be relying on your trinkets to survive anyway.
Remember that your main stat is good defensively in this expansion, and take whatever high item level trinkets with your main stat as the base stat that you can. Know what secondary stats are good for your spec, and try to get trinkets with your main primary as a base and main secondary. Also remember that for survivability, on-use trinkets are generally better than procs. You may want to consider just picking any trinket with base main stat that is good for DPS in your spec.
There is one trinket which is reasonably good for tanking. It’s your main stat + versatility on use:
For Death Knights, Warriors, and Paladins, this trinket is Jes' Howler, which drops from Tol Dagor.
For Druids, Monks, and Demon Hunters, this trinket is Lustrous Golden Plumage, which drops from King’s Rest.
In general, you will want whatever trinkets have main stat as a base, at the highest item level you can find.
Here are a few notable trinkets that you should keep an eye out for, organized by class:
There are a couple of trinkets from world quests which can be reasonably competitive for survivability, but can’t reliably be obtained at a competitive item level. Consider using them if you get them.
Darkmoon Deck: Blockades has been buffed. It’s now quite good for a trinket of its item level. The heal — while small — is a nice bonus, but the primary benefit is the Stamina, which grants an average of 328 (min 72, max 583).
Stamina is less good relative to e.g. Armour or other secondaries for EHP than it was in Legion, but it’s still reasonable. This is still a very over-budget trinket for pure survivability.
For pure survivability this trinket is now bis pre-Uldir (due to the item level cap before its release), and probably one of the better trinkets in general; higher item level trinkets will of course win out, particularly Jes' Howler / Lustrous Golden Plumage.
As always, if you don’t think you need the (relatively minor) survivability gain from using a survivability trinket, consider using a DPS trinket instead.
Darkmoon Deck: Fathoms deserves a special mention because it’s basically just a main stat trinket with no secondary effect, as far as tanking is concerned but is extremely good for damage done. It’s usually much more expensive (and for good reason). You can use both.
I’m sure there will be better ones later in the expansion.