LTS - CI 3 -- how long will it be supported/maintained ?

It's more of a soft reminder so CI won't copy what Laravel is doing to make it (like you said) "similar". I've made my mind and I'll be using 3.1.9 for my next project. From what I see, I don't think CI4 will be fully covered 100% at least by the end of 2019 or early 2020. So, I rest my case... If CI3 becomes an old redundant technology in 2-3 years to come, that's mean PHP 7 will also be obsolete (I don't think so) since PHP 7.2 end of life is expected on 30 November 2020 anyway.

Thank you.

Good luck with your project, I think you're right to go with CI3 since you don't know how long you'll have to wait for CI4 to emerge.  You may well have finished your project by that time.

Also, as someone pointed out to me, even when support for CI3 does eventually come to an end (which looks to be a minimum of two years from now if I read correctly) the framework is well known and pretty light, so you would have the potential to fix any issues yourself if you really needed to.