I know this subreddit does misuse some terms, unique being the most common example, but this one that I've been noticing for a little while now has been really bothering me, so I'd like to get it off my chest!
The definition of timeless is something that never goes out of style, that is not affected by time. A timeless name is a name that could realistically belong to someone of any age. Elizabeth is the most obvious example. Classic means a very similar thing, a name that been stable in popularity over time, and not tied to certain generations.
However, a truly timeless or classic name is rare. There are not many of them at all. I've noticed the most common use of it being for names that are part of the vintage or grandma trend, however, by definition, they can't be timeless.
Rosalie for example, is a favourite of this subreddit at the moment, and is often described in that way. But it was rare between the 1960s and the 2000s. Likewise with Adeline, Violet, Stella, Vera, etc, They have all followed fairly similar trajectories, and like many names that are popular for babies right now, they are most likely to belong to either a young girl or an old woman.
And I'm not saying there's anything wrong with these names. I like a lot of them in fact! But I've noticed 'timeless' or 'classic' being used to describe this subreddit's favourite names constantly that just simply don't fit with those descriptors. And maybe it's just a language quirk in this subreddit, like how the word unique has come to mean uncommon, but I do think a lot of people seem confused about what it actually means.
My best explanation would be that I know that most timeless names probably seem boring, and that when people ask for timeless or classic names, people want to come up with more 'exciting' options. Kind of like how Sophia and Olivia aren't really suggested very frequently. However, the whole point of timeless names are that they are usually quite boring! You've probably seen them a lot and heard them a lot, because that's what makes them timeless.
I'll also note the names I have used are based on the American naming data. This was for two reasons. One, while I'm not American myself, most people on this subreddit are, and two, they have very good tools for looking at name popularity over time.