I am publishing this on hope people who give frequent advice here update it.
The Swiss Travel Pass Flex, which allows a number of usable days (3, 5, 8 or 15) within a month, no longer requires a paper log of used days.
That used to be the case in the past. It also meant your personal printed pass had value, if you lost it, gone were also all the money you spent on it.
Things now have changed, it is possible to buy a Swiss Travel Pass Flex online, receive a voucher by email, which will then allow you to select the days you want to use on this website: https://activateyourpass.com/
A travel day can be selected anytime before travel starts, including on the own intended used day. A QR code will be then generated for inspection.