It's my understanding that "phone" without the ability to tap is illegal in the US.
IIRC, the "must be able to let the govt listen" requirement occurred at the same time as Skpye switched models and the requirement was given as the reason for said switch.
This is not true. Else you couldn't have whatsapp or signal calls (e2e encrypted). In 2009, Skype made the argument with the ACLU that they are not a telecoms carrier, and are therefore exempt from CALEA and similar regulations. Additionally, Skype's original CSEO refused to promise that they did not support wiretaps in the original architecture. We know from Edward Snowden that "old Skype" was included in NSA data sources.
Skype changed architecture because routing calls through neighboring, always-on nodes is not practical in a mobile world where 50%+ of users pay for their data by the MB, and have spotty connections with high latency. For several years before the rearchitecture, Microsoft propped up the network by running all the supernodes itself.
Skype is not Common Carrier. It's an Internet Service which ISP's have been deadset on avoiding getting classified as covered under Common Carrier obligations because it would entail, according to them, enormous harm to the profitability of the sector due to the cost of increased regulatory compliance".
Telco's/Telephony must provide pen register/tap capability. Internet services as of yet need not. Until you become big enough for Federal LE to start harassing.
The relevant regulation in question is CALEA.
So I'm not sure that's accurate that there were any such legal limits on Skype, I'd like to see a citation to more info.