Are there eSIM-based cellular plans suitable for travel (mainly Europe and USA)?

Yes, there is.

You already answered the question yourself, there is a plan suitable for travel from Google Project Fi. I got mine when I was in the US, then left for more than one year then came back. When I was not using the plan I put it in pause, so I would not have to pay the $20 for the basic service while keeping the phone number. I heard that you could even downgrade to Google Voice so you can still have the text and voice for free, albeit only from your browser but it appeared to be a little complicated to set it up. The phone I have does not even have the eSIM to do all this.

If you want a local eSIM for your travel to have a local number, I only know Singtel from Singapore. However they don't make clear if they support devices other than the Samsung Gear S3 LTE. They also don't make clear if they will provide any plan with the eSIM, like a pre-paid.

In theory any operator supporting the Apple Watch Series 3 LTE should also support the eSIM (there is a list here). However, it is also not clear if the operators will accept any eSIM device other than the ones they support.

PS: On a second thought, I've been searching about the advantages of eSIM and I found none for me. The few advantages that may be good for some people is that you can keep multiple eSIM in devices, like those dual SIM phones, except that the limit must be much higher. Other than that, the number of steps and time taken to setup, keep and terminate a cell phone plan seems to be almost the same. Other people may find easier to call the operator to get a eSIM swap (if they speak a language supported by the operator) instead of prying the SIM from the tray. Others may also find good the 10g or so weight reduction of the phone.