Being the only dev in a startup since 2 years without a single day off where I wasn't messaged by my employer I want this. At least I'll have a 3 week out of country trip where I do not bring my laptop later this year...
That's exploitation, no? You're just scammed into it, because you let it slide.
Honestly, that is just bad management. It can make sense if it's your company, but even then, the risk profile is just off the charts. What happens if your only developer leaves or gets sick?
Real engineers think about handling things when stuff goes wrong, not "everything will be on the happy path forever".
Yes, there are constraints, but to me this sounds like an unacceptable level of exposure.
You should really consider another place to work at, unless you own a massive, measurable chunk of the company in a legally binding way.
The only people who should suffer this much are the true busines owners.