Eagle Patch with Explorer/Venturing uniforms for 18-20 year olds were in the 1990s and 2000s. I had it on my Explorer shirt in the early to mid 1990s. When I worked for National Supply, and a de facto Uniform Policeman since our uniforms has to be 100% correct. My employee showed my manager the policy in the Insignia Guide, and he was allowed to wear it on his Venturing uniform.
I think that changed when the old Venturing Bronze, Gold, and Silver awards became Venturing, Discovery, Pathfinder, and Summit ranks.
As I have aged to old coot or goat status, I find few things surprise or tryly annoy me. Not worth the bile. As long as the YP is in place and the Scouter means well and is focused on the youth growing, what is the big deal?
I've seen it a handful of times but what reminded me of this was seeing 2 in one camp, but in different units.
I actually find it helpful, if everyone abides by the square knot rule it's easy to tell who is a late-teens scout vs. a late-teens adult.
I was an Explorer for 4 years and wore the square knot with dark green background. The pocket had the patch shown in my profile pic to the left. I earned Eagle as an Explorer, so never wore the oval rank patch...which was OK with me.
I think the rule might be or might have been that 18-20 is youth in the exploring/venturing/seascout programs so they qualify as youth and youth can wear rank badges.