It must have been love, but [closed]
Grammar can indeed go out of style, but there does not seem to be much evidence that the phrase it must have been love is particularly connected to the 80s—other than the fact that it was the title of one very popular 80s song which was featured in a very popular movie from 1990, Pretty Woman.
Discussion
A well-known exaple of a grammatical construction going out of style is the decline of the subjunctive in British English (see e.g. this discussion at Collins Dictionary). Some more recent examples are listed in this Mental Floss article:1
- a shift from they started to walk to they started walking;
- an increase in the use of the progressive mood;
- an increase in the use of going to, have to, need to, want to;
- an increase in the use of get-passive (e.g. they got promoted).
1The article says that it 'draws on work' by linuistis Mark Davies, Geoffrey Leech, and Christian Mair, but it doesn't give specific references.
Here is another example:
Over the past few decades, three new ways of reporting speech have appeared:
(4) So Karen goes, "Wow - I wish I'd been there!"
(5) So Karen is like, "Wow - I wish I'd been there!"
(6) So Karen is all, "Wow - I wish I'd been there!"
In (4), goes means pretty much the same thing as said; it's used for reporting Karen's actual words. In (5), is like means the speaker is telling us more or less what Karen said. If Karen had used different words for the same basic idea, (5) would be appropriate, but (4) would not. Finally, is all in (6) is a fairly new construction. In most of the areas where it's used, it means something similar to is like, but with extra emotion. If Karen had simply been reporting the time, it would be okay to say She's like, "It's five o'clock,” but odd to say She's all, "It's five o'clock” unless there was something exciting about it being five o'clock.
Is it a lazy way of talking? Not at all; the younger generation has made a useful three-way distinction where we previously only had the word said.
from Is English Changing? by Linguistic Society of America
But I would add that in these (spoken!) contexts, it would sound a bit stilted and formal to actually use the word said.
CGEL records the following two cases of grammatical constructions that may sound old-fashioned to some speakers:
Stative have: lexical or auxiliary (doesn't have vs hasn't)
In AmE stative have always behaves as a lexical verb (and is preferred over have got). In BrE the lexical use has become common too, and the auxiliary use is tending to sound relatively formal or old-fashioned (with have got or lexical have preferred). (p. 113)
and
Non-deontic shall is not wholly restricted to 1st person subjects. Some speakers allow it in 2nd person interrogatives like Shall you take a taxi?, where Will you take a taxi? is likely to be taken as a request rather than a question about your intentions. This use is uncommon and for many speakers sounds old-fashioned and formal; the more usual way of avoiding the request interpretation is by use of the progressive Will you be taking a taxi? (p. 195)
It must have been love
However, as this Google NGramm shows, there is no obvious decline in the use of "it must have been love" in the written English since the 1980s:
Nor is there any evidence on Google NGram for a significant decline in the usage of must have been:
In the Corpus of Historical American English (COHA), there is some evidence that the construction must have been declined in popularity somewhat since the 70s, but has remained fairly constant since the 80s:
decade 1950 1960 1970 1980 1990 2000
words per 39.40 36.03 35.73 28.40 29.10 25.53
million
Having said all this, I have to admit that I don't have access to a corpus that records youth vernacular over time, and perhaps there one might find the tendency mentioned by your student-teacher. But equally possible is that that tendecy was local to that student-teacher's local environment—or that she was simply mistaken.