I'll combine the two points from the comments since neither user decided to propose an answer.
The l'est here is the usual direct object pronoun (le, la, and so on) followed by the verb.
Tu manges mon gâteau ?!
→ Oui, je le mange ... [le = le gâteau]
If you haven't encountered this yet, you could read up on the subject "French direct object pronouns". I did a quick Google search and found a few primers on it, and this one looks pretty good.
These words can also replace the object of être, as le does here:
Tu n'es pas français donc ?
→ Mais si ... je le suis ! [le = français]
Grammatically, this is the exact same as le or la before any other verb. The usual translation would be "it" if replacing a thing, "him" or "her" if replacing a person, or "so" if replacing an adjective.
It's true, though, that this can sound strange in English because we tend to omit the direct object after "to be", as in these examples:
Est-elle la meurtrière ? Is she the murderer?
→ Oui ... elle l'est ! Yes... she is! (literally "she is her / she is it")
→ Mais non, je la suis ! No, I am! (literally "I am her / I am it")
But in French you can rarely leave out le, la, l'. (That can also be seen at the end of this answer.)
So this is what's happening in your sentence:
Si la bise est une pratique courante entre amis, l'accolade l'est beaucoup moins.
→ While kissing on the cheeks is a common practice among friends, hugging is much less so.
This section is overkill for your question, but read on if you're interested.
There's arguably more than one thing that l' could have been intended to mean here. If it refers to une pratique courante, we would probably expect to see a phrase with en ... une ("one of those") instead. If it refers to just the adjective courante, the reader has to implicitly extract that from the noun phrase une pratique courante.
In the absence of that clue, I believe courant(e) is what's replaced, which is why I translated it "much less so" instead of "much less one". The reason I land on this side is that it would seem strange to me to make the jump from an indefinite complement (une pratique courante) to a definite pronoun (le/la).
Edit: The consensus below is that this reading is correct: l' here replaces the adjective courant(e). However, in older or literary French, as Feelew's sources suggest, an indefinite noun phrase could apparently be replaced by a definite object pronoun.
Another question is whether this l' would be le or la if it weren't before a vowel (for example, in the futur simple and passé simple tenses). Common usage today would probably be a "neutral" masculine le when replacing the adjective, and even when replacing the noun if that's still possible, whereas in older/literary French a feminine agreement with either one seems to have been thinkable.