[VMG]

Note that this is much more dangerous than visiting a website. ANSI escape sequences can seriously mess with your system, RCE included.

[kjs3]

Note that this is much more dangerous than visiting a website.

Are you being hyperbolic or do you seriously think the attack surface area of ANSI escape sequences is 'much more' than, say, Javascrpt?

[ifh-hn]

JavaScript has to escape the browser sandbox, does telnet have a similar sandbox? Or can it access the system directly?

I don't know the answer but if telnet can directly access the system that seems more dangerous irrespective of the attack surface.