Templar Tanks have never been super useful for Trial content since they offer very little to a group and can be tricky to play. A large number of groups have always had a Templar Healer meaning they cover all the benefits of the Templar, and another Templar is unnecessary.
Nowadays damage dealers are hybridised and will stack either Weapon Damage or Spell Damage so the one unique buff the Templar has with the Illuminate passive offering Minor Sorcery, might not be needed if you are in a group that stacks Weapon Damage.
In 4-person content Templar Tanks are actually really strong and they are good at Tanking along with supplying additional group healing and support. They don’t have buffs and benefits as strong as the Dragonknight or Warden and they have this additional passive option to provide some group healing.
Sustain on a Templar is ok with the Restoring Focus skill and Restoring Spirit passive. When you get into 4-person content you also have the option to use Repentance which is a huge Stamina sustain skill for fights with a lot of add corpses to use.
The group benefits of a Templar Tank are basically just one passive, but you also have various other group utility for specific situations with the option to provide a Group Purge/Synergy, Group Minor Endurance, Minor Fortitude, and Minor Intellect and Minor Breach from range or when not using Pierce Armor/ One Hand and Shield.
In general, the Templar Tank is fine for 4-person content and for organised Trial groups who need a Templar and don’t already have one.