I wonder if there are a lot of Canadians that simply day hike that trail to see the spectacular Depot Creek Falls, and if some of the signage is really warning about that. BTW - that looks new - don't recall it last time I was there at the Depot Creek TH in 2019.
Another related anecdote: in 2020 I attempted an American side approach via Hannegan Pass and a high route over Taps. I underestimated what I could do on day 1 (hiked all the way to Tapto, then was too beat for day 2). I ended up doing that route in 2021 with a bit more preparation and a longer itinerary. But, the real reason I bring it up is I had considered a second attempt in 2020 and bailed last minute due to rain in the forecast. But one of my partners went for it solo and successfully got all the way to Redoubt Glacier, then got cold feet about his plans out there all alone. He decided to drop down to Depot Creek and hitchhike from the Canadian side back to Sumas rather than retrace his steps back to Hannegan. This was actually and definitely illegal due to COVID restrictions in place. When he got to the border security he spent a few hours explaining WTF had happened but was eventually allowed to cross with no fine or criminal charges or anything. He even asked the Canadian border agents if he'd be welcome back to Canada and they laughed and said "yes".