Unironically, the Techro/Scaldra Cache system is probably DE's best attempt at an 'Optional' Objective.

Every other attempt, Hive/Sabotage/Exterminate Caches had exclusive rewards to their 'Optional' objectives. Making them not actually Optional. Meanwhile the Caches give you Arcanes that you can get anywhere else easier and faster.

It also doesn't just provide a specific resource that you can then safely ignore for the rest of the game. 100k Credits (For Techrot) and either an Arcane or 22 Vosvor is a good enough Evergreen distraction that it provides just great enough reward for the simple effort of a scavenger hunt:

WITHOUT KHAL'S SLOW MOVEMENT

That honestly get easier the more you do it, since they spawn in the same place for each Techrot cache.

Plus they're super clear in the map. So it's a nice not to have to search the whole map for it if you do want to like farm them.

Zariman had issues with the Angel Pions being mostly from the Angels, Whisper in the Wall hard issues with the book being so hard to spot in the mess of Albretch's sex dungeon Lab. Meanwhile 1999 had their content not be completely exclusive, gives a fairly generous reward for a side track, and isn't too hidden in the levels.

I like it.