Feels like I'm back in the year 2004

You can create separations by printing to a PostScript file and use the Distiller PPD that can be found here:

https://helpx.adobe.com/illustrator/kb/save-as-pdf-options-not.html

In the Illustrator Print dialog choose: Printer > Adobe PostScript File.

For PPD select: Other and choose the Distiller PPD: ADPDF9.PPD

Then you are able to select: Mode > Separations (Host Based) in the Output tab.

You can save the PostScript file and use the Acrobat Distiller to create the separated PDF.

But...

It will be a grayscale document that does not contain halftone dots, your printer needs to support printing halftone dots if you want to print anything else than flat solid colors.

The direct download for the PPD:

https://helpx.adobe.com/content/dam/kb/en/852/cpsid_85257/attachments/ADPDF9.zip

So just to put some background on this, we are a pre-press house and adjust client supplied graphics for correct color separations as well as trapping etc. It's only this one client that is requesting this separated pdf for whatever reason as we've done business with them for years without issue. I get the feeling he doesn't have the ability to view separations from a PDF and wants to make sure of correct color seps. Was just posing these thoughts as I wasn't sure if I had missed something out there where this can be easily done. Thanks for all the responses guys.

That doesn't really make much sense and even in 2004 would have been one heck of a non-ideal workflow. It's really not advisable to provide separations outside the facility's internal loop where they have tweaked settings and color profiles to their printing machines. I haven't seen anyone request PS files or individual separations in over a decade (except for specialized spot color like processes like textile printing).

Mylenium

Our client is looking to get a PDF file that contains separate pages for each separation. The way this was done back in the day was to print the file from your application of choice to a postscript file (using print as separations), then distill it using Acrobat Distiller to convert that to a PDF. It's been ages since I've needed to do this and I just wonder why anyone would even want this as an option.

Do you mean the client wants separate CMYK files with the individual channels as a separate document?

Separations are separations, with or without transparency, so I'm not sure what you're specifically asking. You'd simply use AI's separations preview or check the file in Acrobat based on what you created. The only real caveat is to have the color management set up correctly as per the client's requirements. I'm also not sure why you would even want to distill a EPS/ PS file when most RIPs these days natively support PDFs and/ or they get converted to the native printer language via the driver. In that regard PostScript is pretty much dead as a delivery format and only used for intermediate workflows in the print facilities themselves, at least in reasonably modern ones.

Mylenium