Do leap seconds even matter here? Doing anything involving DNS or certificates in a way that requires clock synchronization down to the second would seem to be asking for trouble.
Basically when it was invented leap seconds seemed like a good idea because we assumed the inconvenience versus value was a good trade, but in practice we've discovered the value is negligible and the inconvenience more than we expected, so, bye bye leap seconds.
The body responsible has formal treaty promises to make UTC track the Earth's spin and replacing those treaties is a huge pain, so, the "hack" proposed is to imagine into existence a leap minute or even a leap hour that could correct for the spin, and then in practice those will never be used either because it's even less convenient than a leap second - but by the time they're asked to set a date for these hypothetical changes likely the signatory countries won't exist and their successors can just sign a revised treaty, countries only tend to last a few hundred years, look at the poor US which is preparing 250th anniversary celebrations while also approaching civil war.
If humans cared the existing systems couldn't exist. For more than a century we've all lived with time "zones" which are thousands of seconds wide and we're not bothered by that. Many of us have civil time systems which shift twice per year by 3600 seconds for really no good reason, and while that's annoying it's barely worth a brief mention on TV news or in small talk. Leap seconds are 3600 times smaller and happen way less often, they're entirely negligible.
They existed because we thought we cared, and we actually don't care, and we thought it was pretty easy to do, and it actually wasn't very easy after all.
The leap second is because the Earth's spin varies. The orbit isn't varying significantly, it's just not a whole number of day-night cycles which is inconvenient for us, but the spin actually varies. To "fix" this variation we have leap seconds. But unlike the very noticeable October versus November difference, a century of missed leap seconds adds up to much less than a minute of difference between solar time and civil time, and that's not something you'll actually notice.
In contrast leap seconds are not predictable because they're trying to smooth out spin variation and the planet's spin is affected by natural processes. The IERS measures UT1 and we add (or remove) leap seconds to make UTC try to approximate UT1, we get a few months warning at most that a new insertion or removal is needed.
And despite being very difficult all of this is also pointless because we don't care. As an astronomer you know you don't try to use a wristwatch to figure out the Earth's rotation, you can just look that up online and use your exact position. But for any other purpose except astronomy we don't need this angle information, we want a nice regular time and the atomic time, TAI, already provides that without this nonsense about leap seconds.
unixtime is almost certainly what is meant by the standard, but it is not the count of UTC seconds since 1970; unix time is the number of seconds since 1970 as if all days had 86400 seconds. UTC, TAI, and GPS seconds are all the same length, and the same number have happened since 1970, but TAI appears 37 seconds ahead of UTC because TAI has days with 86400 seconds, while UTC has some days with 86401 seconds and was 10 seconds ahead of UTC in 1970. unixtime and UTC are in sync because unixtime allows some days to encompass 86401 UTC seconds while unixtime only counts 86400 seconds.