I tried to find the answer on the web, but there seems to be no information at what point old piBase extensions are going to stop working in TYPO3.
The transition of the "old" TYPO3 in direction of FLOW is visible, but I have to give a client an assessment of how long he will be able to use a piBase extension he had custom programmed.
With the current 6.2 LTS Version he should at least be save for the next two years, but does anybody know more about the plans regarding piBase?
P.S.: My apologies if there is an article somewhere explaining this that I didn't find.
Best How To :
Concluding this discussion http://typo3.3.n7.nabble.com/TYPO3-6-2-which-Extensions-will-still-work-td244691.html you don't need to afraid that will be dropped soon, anyway...
Keep in mind that next versions of TYPO3 rises PHP requirements, introduces depreciated and/or removed functions etc. Therefore if author of some extension won't fix it to fit new requirements it can stop working even tomorrow. That happened many times before and will happen in the future - for sure. Lot of extensions written let's say ten years ago doesn't work even in 4.x branch.
It's rather obvious, that if you/client want to continue with newest stable versions of TYPO3 you need to prepare to massive debugging of old fashioned extensions and also rewrite some part of them.
Finally if some TYPO3 instance works well on 4.x you don't need to rise it only because there's new polished 6.x in such case I'd recommend to leave it "as is" and develope new functionality with newer versions avoiding next piBase exts.