correct, understanding the instruction set architecture you are working with is required for reasoning about the performance of a given algorithm in detail.

you will likely not be writing a lot of assembly by hand, however steering the compiler codegen in the right direction requires an understanding of what the compiler produces.

even outside of enhancing performance, knowledge of instruction sets is instrumental for security research and reverse engineering. for some fun but practical demonstrations, see work by Nathan Baggs on YouTube - it involves staring at a lot of disassembly.

i don't know where this misguided notion that assembly language is "1975" comes from. it's not like Cobol where a few large but important systems keep it alive. this is something that lies at the core of every interaction with computers that you have daily.