Income averaging was generally available from 1964 to 1986. Since 1986, it can be used only by farmers and fishermen. https://www.irs.gov/pub/irs-soi/87inar.pdf mentions the repeal.
Income averaging was repealed for 1987.
This IRS web page describes income averaging for farming:
If you are engaged in a farming business, you may be able to average all or some of your current year's farm income by shifting it to the 3 prior years (base years).
(Presumably there is a similar reference for fishing.)
Schedule J is used to elect income averaging.
The instructions for schedule J are here: https://www.irs.gov/pub/irs-pdf/i1040sj.pdf
Use Schedule J (Form 1040) to elect to figure your 2017 income tax by averaging,
over the previous 3 years (base years), all or part of your 2017 taxable income from
your trade or business of farming or fishing. This election may give you a lower tax if
your 2017 income from farming or fishing is high and your taxable income for one or
more of the 3 prior years was low.
A farming business
is the trade or business of cultivating
land or raising or harvesting any agricultural
or horticultural commodity.
A fishing business is
the trade or business of fishing in which
the fish harvested, either in whole or in
part, are intended to enter commerce or
enter commerce through sale, barter, or