When I look at Classical music artist-recording relationships it seems usually the ‘solo’ attribute is not checked for individuals credited as musicians/vocalists even when I would consider them as soloist (such as a subcategory of lead vocals)
Would I be correct to say they are underused and are there any style guidelines about this, I couldn’t see anything.
I seem to recall that the solo attribute is only to be used if they are specifically credited as a soloist in so many words somewhere on the release. However it has been a while since I looked into that, so the wording may have changed.
If so would it be fair to say its just a representation of whats written on the cover then rather than the reality of the situation then ?
Not really the cover but anywhere in the credits.
If I recall the intent is that any “solo” part be given the attribute but it becomes extremely difficult what is specifically a “solo”, e.g. you wouldn’t use it on a piano credit for a piece that was only for piano and nothing else.
Sure, but if you take the case of a piece performed by an orchestra and some individual performers are credited should they not (nearly) always be considered as soloists.
Or if you have choral piece performed by a choir, and tenor and sporano vocals are credited individually should they not always be considered solosts.
Yes, exactly but, as @Hawke says, not only the cover.
We can consider anything we want but the MB relationships are made to reproduce the package texts.
So if it’s printed solo somewhere, including in another release using this recording, we start checking that checkbox (except if it is a mistake). 
We have enough work to do with only that to start doing more, haven’t we ? 
I suspect the reason for the limitation is to be certain that the solo is of sufficient importance or length within the piece to justify calling it out as something noteworthy. If the harmonica player has two notes during a brief period when no-one else is playing then I wouldn’t consider that to be worth calling a solo for MB purposes even if it could by stretching a point be called one.
I don’t think a classical piece would give such a credit in the first place would they, i mean I’m not talking about inventing credits I’m just wondering when they are credited would it not be the case in some situations they are a soloist but the word ‘solo’ does not appear because its obvious.
I certainly see the word solo or soloist more commonly on popular music, but I have definitely seen some examples in classical, such as credits for clarinet soloist, trumpet soloist etc. I do think it is less common to use the term in classical in the credits.
I’ve used solo for the soloist in a concerto before, where there’s little doubt it does indeed apply. I would be a bit more cautious using it for a non-concerto piece for, say, piano and orchestra, but really, whether a performance is a “proper” solo or not probably depends as much on the piece as on what’s printed (and I’d definitely still think it’s fine to use “solo” for a violin concerto even if the release prints just “violin”).
I would say yes but the decision was made to defer to the packaging/credits in order to avoid ambiguity.
Could something be added to the Style guidelines to make the position clear, whatever it is.
I have edited today a jazz release. The artists credits on the back cover is generally but after I listened the album I recognized all the instruments (the artists played all the tracks) and there is a combination of solo playing every track… so, should I add them as solo instrument track by track?
This is my edit:
Yes. There is key difference between classical practice (topically discerning an exceptional solo from the regular, titular and thus obvious soloist) being fuzzy from pragmatics, and MusicBrainz’ accuracy standard: If you will, anti-alias vs. exacted ID. Stems from classical concept of form traditionally expressed through classifying titles (symphony, sonata, suite, fugue, …) differing from popular tradition where less focus on form leaves no need for classifying (main form: song) but room for poetic reflection (title typically lyrical).
Some well-known examples of solos often specially credited:
Bach: [Many vocal movements with obbligato instrumental solos]
Beethoven: Missa solemnis: IV. Sanctus; Benedictus (Violin solo)
Berlioz: Symphonie funèbre et triomphale: II. Oraison funèbre (Trombone solo)
Schumann, Clara: Piano Concerto: II. Romanze (Cello solo)
Brahms: Piano concerto 2: III. Andante (Cello solo)
Brahms: Violin Concerto: II. Adagio (Oboe solo)
Saint-Saëns: Symphony 3 (Organ solo)
Strauss, Richard: Don Quixote (Viola (, tuba & bass clarinet) solo)
Strauss, Richard: Ein Heldenleben; 3. The Hero’s Companion (Violin solo)
Shostakovich: Piano Concerto 1 (Trumpet solo, verging on double concerto)
Ives: The Unanswered Question (Trumpet solo)
Pettersson: Symphony 16 (Saxophone solo verging on concerto)
Messiaen: Symphony Turangalîla (Ondes Martenot solo)