An old employer did this, but instead of just at the beginning this was a monthly rotation thing (3 days each month or so). Being forced to do support definitely was a major factor in my quitting. I don’t even mind support so much, used to do it for a job in university. But you don’t hire a highly educated professional and stick them in an entry tier role part of each month, not if you respect them.
In theory this sounds like a good idea. In practice, it was a poorly thought out system that left me frustrated with the company, disconnected from my team, and looking for a new job before I even finished the half implemented on boarding process. It felt more like a hazing ritual than a useful learning process and completely soured me on the company.
Doing support isn't something I find particularly egregious, but you should absolutely not segregate new hires away from their team for a long period of time immediately after hiring them. More generally, you need to equip new hires to do their job as soon as possible, less you risk smothering their enthusiasm for what they were hired to do.