The update for 1.13 is being worked on, but there is no release date for it yet.
(12/20/18 Update): The goal is to have a working build available by New Years.
(02/14/19 Update): Well obviously we were unable to make our goal of having a release by New Years. Again this was a GOAL, not a promise, please DO NOT ask for ETAs, threads requesting an ETA will be locked.
(2/19/19 Update): While it's not "finished" yet by any means, in the interest of getting more feedback and bug reports versions of Forge for 1.13.2 are now available on the Files page, as ever please back up any worlds you care about before you even think about playing them in these beta versions.
Updates to a new version of Minecraft take a lot of work, so please be patient.
(12/20/18 Update): In addition, the Dev team has taken this opportunity to update and retool right down to FML. This means making things more future-proof, partly by cleaning out/replacing a TON of legacy code that has been there since the early days and is showing its age.
Please refrain from making "is it done yet?" threads. The update will be ready when it is ready.
(12/20/18 Update): For those of you who insist on asking for an ETA, the most accurate estimate can be made by looking at what patches still have to be ported.
In the meantime, DO NOT fall for sites offering Forge for 1.13, these sites will NOT magically provide you with software that has not been released yet, they will more likely install malware on your computer.