The earliest Greek manuscripts up to at least the 6th century use all capital letters and were known as "uncial" or "majuscule" manuscripts. However, the 6th century saw the first signs of the early development of miniscule script that was fully developed by about the 9th century. It is only in this later miniscule manuscripts that there is any distinction between capital and lower case letters. Between the 6th and 9th centuries, manuscripts were produced in both forms in different places.
For the sacred proper names like "God", "Jesus", "Messiah", "Spirit", "Lord", "Father" and a few others, there was a practice dating from the very earliest manuscripts of the second century a system of abbreviation showing that the words represented sacred proper names. Here are a few. The abbreviation was highlighted by an overbar drawn atop the letters (not reproduced below). These were known as "nomina sacra".
God -> Θεός -> ΘΣ
Lord -> Κύριος -> ΚΣ
Jesus -> Ἰησοῦς - ΙΣ
Spirit/Ghost -> Πνεῦμα -> ΠΝΑ
Savior -> Σωτήρ -> ΣΗΡ
Thus, if we follow the earliest manuscripts (at a time before the formal doctrine of Trinitarianism/Arianism/Modalism/etc even developed) we see that the word "Sprit" in numerous places was regarded as sacred.
For more information see https://en.wikipedia.org/wiki/Nomina_sacra
In modern translations since about 1960, capitalisation of "Holy Spirit" is more restrained than in the earliest manuscripts.
The Greek phrase "pneuma hagios" (= Holy Spirit) occurs 90 times in the NT. Here are the first few: Matt 1:18, 20, 3:11, 12:32, 28:19, Mark 1:8, 3:29, 12:36, 13:11, Luke 1:15, 35, 41, 67, 2:25, 26, 3:16, 22, etc. (The writer Luke uses this phrase more than any other NT author and more than all other NT authors combined.)