I believe the reference is to human spirits, for at least three reasons:
#1 People who died in the flood
I believe the 1 Peter passage makes a straightforward reference to people who died in the flood—it is helpful to consider verse 20 in its entirety to capture the contrast:
Which sometime were disobedient, when once the longsuffering of God
waited in the days of Noah, while the ark was a preparing, wherein
few, that is, eight souls were saved by water.
The fate of Noah and his family is contrasted with that of the people being discussed just before—these are people who were disobedient during Noah’s day and were not saved in the ark.
This then sheds light on the passage from 2 Peter 2:
4 For if God spared not the angels that sinned, but cast them down to
hell, and delivered them into chains of darkness, to be reserved unto
5 And spared not the old world, but saved Noah the eighth person, a
preacher of righteousness, bringing in the flood upon the world of the
6 And turning the cities of Sodom and Gomorrha into ashes...
He’s citing 3 different examples to establish a point—that God can and does punish sin—and since those who died in the flood are separated in Peter’s list from the angels in verse 4 the same way Sodom and Gomorrha are separated from those who died in the flood, the logical conclusion is that these are three different groups. (Otherwise we’re left trying to combine all three groups and explain how the wicked at Sodom & Gomorrha are the same as the people who died in the flood).
Peter’s examples also appear to be listed in chronological order. This is further substantiated if one holds (I do) that Isaiah 14:12-15, 2 Peter 2:4, Jude 6 (see also the Book of Enoch quoted by Peter & Jude), teach about the fall of Lucifer (and those who followed him) before the earth was created.
Preaching to the dead is made more explicit in 1 Peter 4:6:
For for this cause was the gospel preached also to them that are dead,
that they might be judged according to men in the flesh, but live
according to God in the spirit.
#2 Disobedient long ago…in the days of Noah
“[T]o those who were disobedient long ago when God waited patiently in the days of Noah” would be a very straightforward way to refer to people who were disobedient and died in the flood; it would be a very convoluted way to refer to beings who had been disobedient on the earth before that time, during that time, and since.
As noted above in the discussion of the parallel passage in 2 Peter 2, the angels in Jude 6 appears to be a reference to an event prior to the flood. The evil spirits in Matthew 8 may well be referencing the judgement of Rev 20, but I do not see any reason to tie them to the individuals in 1 Peter 3:19-20. The evil spirits of Matthew 8 were clearly still being disobedient in the present, and would not be referred to as “disobedient long ago…the days of Noah.”
#3 The angel/human distinction
This one is more general; but at least tangentially interacts with the OP’s questions. I am not persuaded that a clear angel/human distinction is warranted by the Biblical texts. As noted by Gina here, the Hebrew “malak” and the Greek “aggelos” mean “messenger”, which can have a wide variety of applications. See also Jay Adam’s comments here regarding the influence of mythology on how some texts about angels may have been misread.
I recognize that there are those who for a variety of reasons do not believe that the dead are conscious—some of my thoughts offering the opposite perspective are found here.
I suggest the dead are conscious as disembodied spirits and, absent a compelling reason to believe otherwise, texts such as 1 Peter 3:18-20, 1 Peter 4:6, and probably Isaiah 9:2 as well, which refer to preaching to the dead, say what they appear to say: the Gospel is preached to the dead. This then suggests the proclamation of 1 Peter 3:19 is not one of the arrival of condemnation, but one of good news (the meaning of “gospel”).
As I argued here, I understand that salvation is to be offered to all whether in this life or the next (doesn't say all will accept it). Some in the days of Noah apparently had no such opportunity in life.