Jo længere bindingsperioden er, desto vigtigere er opsigelsesfristen og abonnementsvilkårene.

No. It is the same spelling, but a different use.

In this sentence, jo is part of the pattern jo ... desto ..., which expresses a comparison of change:

Jo længere ..., desto vigtigere ...
= The longer ..., the more important ...

So this jo does not mean yes or after all. It works more like the English the in the longer ..., the more ....

It creates a correlative comparative: one thing increases or changes together with another.

Structure:

jo + comparative + clause, desto + comparative + clause

In your sentence:

• jo længere bindingsperioden er = the longer the binding period is
• desto vigtigere er opsigelsesfristen og abonnementsvilkårene = the more important the notice period and subscription terms are

So the whole idea is:

As X becomes more ..., Y becomes more ...

This is a very common Danish pattern.

Yes, often you can.

Many speakers use jo ... jo in everyday Danish:

Jo længere bindingsperioden er, jo vigtigere er opsigelsesfristen og abonnementsvilkårene.

That sounds natural too.
Very roughly:

• jo ... desto can sound a bit more formal or written
• jo ... jo is very common in speech and informal writing

Both are worth recognizing.

Because both are comparatives.

• lang → længere = long → longer
• vigtig → vigtigere = important → more important

So:

• jo længere = the longer
• desto vigtigere = the more important

A useful detail: længere is not formed regularly from lang. The vowel changes, so you need to learn it as a form on its own.

Also, Danish often makes comparatives with -ere, where English may use either -er or more.

Because vigtigere is the normal comparative form of vigtig.

Danish can sometimes use mere + adjective, but for many common adjectives the -ere form is preferred. Here, vigtigere is the standard, natural choice.

So:

• natural: vigtigere
• much less normal here: mere vigtig

Think of it like English preferring more important rather than trying to build an -er form.

Because the sentence is talking about the specific contract terms under discussion, not just such things in general.

Forms:

• en bindingsperiode → bindingsperioden = the binding/commitment period
• en opsigelsesfrist → opsigelsesfristen = the notice period
• et abonnementsvilkår / plural abonnementsvilkår → abonnementsvilkårene = the subscription terms

This is very natural in consumer-contract language. The sentence is not saying a notice period in general, but the notice period and the subscription terms of that agreement.

Because the sentence has two clauses, and each clause has its own verb.

1. Jo længere bindingsperioden er
2. desto vigtigere er opsigelsesfristen og abonnementsvilkårene

Both clauses are built around er = is/are.

English does the same in a full version:

The longer the commitment period is, the more important the notice period and the subscription terms are.

So the repeated er is completely normal.

Because Danish main clauses follow the V2 rule: the finite verb usually comes in the second position.

Here, the first position is taken by the phrase desto vigtigere. That means the verb er must come next:

• Desto vigtigere | er | opsigelsesfristen og abonnementsvilkårene

So the subject comes after the verb.

This kind of fronting is very natural in the jo ... desto ... pattern, because the second half mirrors the first half nicely:

• Jo længere ...
• Desto vigtigere ...

If you started with the subject instead, the sentence would feel less parallel and less elegant.

Because Danish normally writes compound nouns as one word.

abonnementsvilkårene is built from:

• abonnement = subscription
• linking -s-
• vilkår = term/condition
• definite plural ending = -ene

So:

abonnement + s + vilkår + ene
→ abonnementsvilkårene

This is very typical Danish spelling. English often writes such things as separate words, but Danish usually joins them.

The same thing happens in:

• bindingsperiode
• opsigelsesfrist

These are also compounds.

Because in Danish, comparative adjectives do not agree with gender or number.

So vigtigere stays the same whether the noun is:

• singular
• plural
• common gender
• neuter

Compare this with the positive form:

• en vigtig regel = an important rule
• et vigtigt punkt = an important point
• vigtige regler = important rules

But in the comparative, it is simply:

• vigtigere

So even with the plural subject opsigelsesfristen og abonnementsvilkårene, vigtigere does not change.