I am looking for a word or phrase that describes something that was very modern in the time being referenced, even though it is clearly not modern now. For example, I am writing about a computer language from the 1970's.
Involving new ideas or methods:
Company X used FORTRAN, a pioneering programming language [of the 70's].
'Pioneering' provides, or implies, the context of historical foundation and excitement of the evolving technology of programming languages, FORTRAN in particular, during the 1970's.
As pointed out in comments, state-of-the-art is an apt word.
the latest and most sophisticated or advanced stage of a technology, art, or science.
Usage: X company used FORTRAN, a state-of-the-art language in the 70's
If you are looking for alternatives, then you may consider :
ahead of the times; advanced
Usage: X company used FORTRAN, a futuristic programming language
given to or characterized by fanciful, not presently workable, or unpractical ideas, views, or schemes
Usage: X company used FORTRAN, a visionary programming language, which continues to be used for scientific computing
Meaning Source: Dictionary.com
'Innovative'. From the Oxford English Dictionary:
[ad. L. innovātiōn-em, n. of action f. innovāre to innovate: cf. F. innovation (1297 in Hatz.-Darm.).]
1. a. The action of innovating; the introduction of novelties; the alteration of what is established by the introduction of new elements or forms. Formerly const. of (the thing altered or introduced).
innovate innovate, v.
[f. L. innovāt-, ppl. stem of innovāre to renew, alter, f. in- (in-2) + novāre to make new, f. novus new. Cf. F. innover (1322 in Godef. Compl.).]
2. To bring in (something new) the first time; to introduce as new. Obs. exc. in Comm.
'1548 Udall Erasm. Par. Luke Pref. (R.), If any other do innouate and brynge vp a woorde to me afore not vsed or not hearde, I would not disprayse it. 1592 R. D. Hypnerotomachia 57 b, This..table being taken up..there was presently an other innovated, with a cloth of silke. 1656 Burton's Diary (1828) I. 98 He was tried in the same way for innovating a new religion. 1666 Dryden Pref. Ann. Mirab. Wks. (Globe) 41 Some words which I have innovated..upon his Latin. 1735–8 Bolingbroke On Parties 63 To surrender their old Charters, and accept new ones, under such Limitations and Conditions, as the King thought fit to innovate. 1967 Times Rev. Industry Oct. 86/2 Nylon..was first invented in 1928, but not innovated until 1939. 1972 Physics Bull. Feb. 67/1 (Advt.), We've been innovating electrometer values like these for over 20 years.'
If you rely on a context that has already informed your audience of the particular period, or a context that will make that plain, then
2. The pioneers or innovators in any art in a particular period. Also attrib. or as adj.
["avant-garde | aˈvant-ˌguard, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/13610?redirectedFrom=avant-garde (accessed December 11, 2015). Emphasis mine.]
For example (from the comments), without clauses,
Company X used the avant-garde FORTRAN language.
won't be misunderstood. With a clause,
Company X used FORTRAN, an avant-garde language.
You could consider cutting edge:
the sharp edge of something that is used to cut things
the newest and most advanced area of activity in an art, science, etc.
the most recent stage in the development of something: His research is at the cutting edge of new therapies for cancer.
very modern and with all the newest features: cutting-edge design/technology
Company X used FORTRAN, a computer language that was the thing back in the 70s.
Also, the in thing. Something in style or vogue, the latest trend. The first term dates from the mid-1700s; the second, which uses in in the sense of "popular," dates from the mid-1900s. American Heritage® Dictionary
adjective: relevant or applicable to a particular matter; apposite:
he asked a lot of pertinent questions: relevant, to the point, apposite, appropriate, suitable, fitting, fit, apt, applicable, material, germane, to the purpose, apropos; on-topic; formal ad rem.
src. apple dictionary