Can OpenWrt packages be installed on an earlier release? I'm guessing this generally isn't a good idea, but maybe it's safe for some packages?
Specifically, I'm using 19.07, and I'd like to try out some stuff with gnuplot, which is only available on 21.02. Can I take that 21.02 ipk and install it on 19.07? Or is it likely to cause problems? Will opkg even let me install it?
Generally, no.
Moderately likely.
The C library (musl libc) used in the compilation typically changes between releases, and the executable binary is pretty much tied to that. 19.07 and 21.02 use roughly the same version, so there is some hope. (but as the versions are not quite fully identical, a failure is likely.)
Similarly, also the other libraries it possibly links to, may have changed between releases.
So, quite likely the .ipk causes problems. In some cases it might even work, but that is not the high-probability expectation.
(Typically something like "ash: COMMAND not found" style of errors visible in shell, when faulty binaries are tried.)
Thanks, I just went ahead and tried it (using the ipk from here), and it works fine.
Time to start playing! 
Looks like it found all its dependencies in the 19.07 repo:
Installing gnuplot (5.4.1-1) to root...
Installing libjpeg (9c-2) to root...
Downloading http://downloads.openwrt.org/releases/19.07.6/packages/x86_64/packages/libjpeg_9c-2_x86_64.ipk
Installing libpng (1.6.37-3) to root...
Downloading http://downloads.openwrt.org/releases/19.07.6/packages/x86_64/packages/libpng_1.6.37-3_x86_64.ipk
Installing libgd (2.2.5-2) to root...
Downloading http://downloads.openwrt.org/releases/19.07.6/packages/x86_64/packages/libgd_2.2.5-2_x86_64.ipk
Configuring libjpeg.
Configuring libpng.
Configuring libgd.
Configuring gnuplot.
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