I am traveling as a Belgian citizen after 28 March (brexit). Can I still enter with my identity card?
Yes you can. Per a policy paper, in case of no-deal, EU IDs will be accepted until at least 2021. After that, it depends on what the government deicdes then.
Membership in the EU is not a requirement for accepting identity cards. As an example, Turkey accepts Belgian identity cards despite not being an EU member. The UK itself likewise accepts ID cards from Monaco, which is not an EU member.
So there is an extremely high probability that you'll be fine at least until the end of the year. There's absolutely no reason for the UK to stop accepting ID cards all of a sudden.
I think everybody wants it to be simple for EU citizens to travel to the UK (and the other way), but as far as I have heard they haven't made any official agreements yet (I'm sure there's not a complete solution to all of the issues brexit raises, but not if any agreements have been made that would cover this or other small areas, to prevent some chaos), and without one you won't be able to.