tz@iana.org said:

I see that RFC 8633 cautions against public-facing servers' using leap smearing, and NTP clients' using a mixture of smeared and unsmeared. I suppose the same should apply to quasi-NTP UT1 servers such as:

Correct, but the nature of the errors is different from smearing servers. There are many smearing servers. There is no way in the protocol to tell if you are talking to a server that will smear when a leap second comes along. The whole point of them is to trick innocent clients into avoiding leaps. If you use a UT1 server, the error will likely be visible right away. If you happen to catch it near the zero crossing, it will soon drift off. There aren't many UT1 servers. You are very unlikely to use one by accident. I don't think it's worth cluttering up TZ info with a warning about UT1 servers. It might be useful as an example of a more general warning. -- These are my opinions. I hate spam.