ИНН и НДФЛ указаны здесь.

These are common Russian abbreviations, especially in tax and document-related contexts:

• ИНН = Идентификационный номер налогоплательщика
  This is the taxpayer identification number.

• НДФЛ = Налог на доходы физических лиц
  This is personal income tax.

In real Russian, abbreviations like these are used very often, especially on forms, websites, and official documents.

Because the sentence has two things as its subject:

• ИНН
• НДФЛ

Together they form a plural subject, so the predicate is plural too.

That is why Russian uses:

• указан = singular masculine
• указана = singular feminine
• указано = singular neuter
• указаны = plural

So ИНН и НДФЛ указаны здесь literally means something like The INN and the NDFL are indicated here.

Указаны is the short passive participle of указать.

In practical terms, learners can think of it as a form often used like are indicated / are specified / are listed.

Compare:

• указать = to indicate, specify
• указан / указана / указано / указаны = indicated, specified, listed

This short form is very common in formal, written, and official Russian.

Because Russian usually does not use a present-tense equivalent of to be in sentences like this.

English says:

• The tax ID and income tax are indicated here.

Russian simply says:

• ИНН и НДФЛ указаны здесь.

This is normal Russian grammar. The idea of are is understood automatically.

They are in the nominative because they are the grammatical subject of the sentence: they are the things being described as indicated here.

A rough structure is:

• ИНН и НДФЛ = subject
• указаны = predicate
• здесь = adverb meaning here

So this is like:

• The INN and NDFL are listed here.

Yes, Russian word order is flexible.

The version you have:

• ИНН и НДФЛ указаны здесь.

is completely natural.

You could also say:

• Здесь указаны ИНН и НДФЛ.
• ИНН и НДФЛ здесь указаны.

These all mean basically the same thing, but the emphasis changes slightly:

• Здесь указаны ИНН и НДФЛ puts more focus on here
• ИНН и НДФЛ указаны здесь sounds neutral and natural
• ИНН и НДФЛ здесь указаны can sound a bit more contrastive, as if here is being emphasized

It sounds somewhat formal / administrative / written, which is very normal for forms, websites, instructions, and official documents.

That is mainly because of указаны, which is a typical formal word in Russian.

In casual speech, people might choose simpler wording depending on context, but on a document or page this sentence is very natural.

They are usually pronounced letter by letter, not as normal words.

• ИНН is pronounced like the names of the letters: и-эн-эн
• НДФЛ is pronounced: эн-дэ-эф-эл

This is common with many Russian abbreviations.

Both forms can be possible in some contexts, but they are not exactly the same in style.

• указаны = are indicated / are listed
  This is concise and common in official text.
• указываются = are being indicated / are stated / are specified
  This can sound more procedural or descriptive.

In signs, forms, and interface text, указаны is often the more compact and natural choice.

Normally, with just two items, Russian uses и:

• ИНН и НДФЛ

A comma would not normally replace и here.

A comma is more typical in a list of three or more items:

• ИНН, КПП и НДФЛ

So in your sentence, и is exactly what you would expect.

Yes. Even though ИНН and НДФЛ are abbreviations, Russian still treats them as nouns for agreement purposes in the sentence.

Since there are two coordinated items, the predicate becomes plural:

• ИНН и НДФЛ указаны

So the fact that they are abbreviations does not prevent normal grammatical agreement.