Can you book a trip 2 years in advance with a travel agent? (You can’t book that far online).

Some trips you have too, like white water rafting the Snake River through the Grand Canyon is a five year wait.

I don't think so as they can't estimate costs etc. Best to book closer to the time

What happens if the travel agency goes under

Given how retail in general is struggling to maintain a presence i don't think any company can guarantee anything that long-term.

If they do you'd need to ensure what they were offering was covered by a recognised money back guarantee scheme

Depends what kind of trip. Airlines usually only open their booking engine 360 days in advance and if you want to include travel by air, this will not be possible.

Also, most OTAs (online travel agents) don't allow bookings more than 11 months ahead of time.

How can you guarantee that all the logistics for a trip will remain the same that far out? Plenty of people planned trips in 2018 and 2019 for 2020 and onwards then the Covid plandemic ruined it all.

I think so theres prolly a minimum buy in or like pay into in advance kinda thing.

Go and find out

It's actually not recommended, so many things could happen between then, plus too many bookings, both agent and online, have super fine print that have stipulations that prevent refunds.