If they did upgrade all their actives then 3.1 upstream is likely in the works. DOCSIS has some pretty amazing diagnostics and troubleshooting - I can look at a modem and say "approximately 71 ft from the modem there is a break in the cable causing an RF impairment" or "These 14 modems in the same neighborhood show the same impairment - there is likely a fault at this specific segment of cable". DOCSIS 3.1 expands on that even more and as more 3.1 CPEs get deployed operators are able to leverage that additional data to get the plant ready for 3.1 upstreams. OFDM frequencies are also not backwards compatible with 3.0 CPEs but a 3.1 CPE can bond both types of channels, so as the ratio of 3.1 CPEs increases that allows operators to shift more frequencies from legacy modulation to newer 3.1 modulation.
How is 300/15 better than 1000/50?
edit: the ratio is the same, I just haven't powered up my brain yet.
Most of their passive and active equipment in areas which have been fully "digitalized" is 3.1 compliant, they just need to finish the whole city and then they will start marketing the offer. Modems will have to be changed as well as equipment on the customer's side which they haven't done yet, so it will take quite some time.
They "promised" it by the end of 2019. (in 2016.), seems it will be 2025. By that time the Chinese government and Telekom Srbija will have FTTH to every home.