Just curious, will the old functions be removed entirely??
Old functions will be deprecated, but it's unlikely we'll be able to fully remove them for a long time, given the number of projects that still use them. In particular projects still using the C2 runtime can't switch to builtin functions until they switch to the C3 runtime. So realistically we could only fully remove the old functions if we fully remove the C2 runtime and pretty much all C3 runtime projects have switched over to using builtin functions. That is probably at least a couple of years off.
Tokinsom - I'm currently investigating a feature to switch old functions to builtin functions for the next beta. The main complication is old function parameters could be numbers or strings, but builtin functions are typed and must be one or the other. So if the parameter uses both types it cannot be converted. Additionally doing things like calling functions by expressions throws off the static analysis and forces it to assume that call could call any function in the project, making it more likely it can't work out the parameter types. So even if I can get it working, it's not yet clear it will be useful in practice, depending to the extent people's projects use dynamic types or calling functions by expressions.