I'm sorry, I think you've misunderstood the point of the Eszett spelling reform.
The goal was to somewhat codify the idea that a single consonant (e.g. the "l" at the end of "Tal") implies that the previous vowel should be long (t-ah-l) while a double consonant (the "ll" at the end of "Ball") implies that the previous vowel should be long (b-a-l). (This was already true most of the time for other consonants, but "s/as/ß" remained an exception.)
Therefore similarly, by analogy, "Fass" has a double consonant at the end of the word, so we know that it is a short vowel (f-a-ss), and "Maß" has a single consonant at the end, so we know it must be long (m-ah-ss).
As an aside, this doesn't explain why we still need ß in the first place - if it's just there to indicate long and short vowels, why not just use a single "s" like with the other consonants? The answer is in pluralisation rules: when you pluralise "Maß -> Maße", you still say a hard "s" sound (m-ah-ss-uh). But if you pluralise, say, "Los -> Lose", the "s" becomes a "z" sound (l-oh-z-uh). In German orthography, it's generally the case that you need to know both the single and plural of a word to be able to spell it correctly.
Even under this new scheme, a lot of short, very common words retain their original spelling, which is why "das", which was an irregular, common word under the old scheme, remains an irregular, common word under the new scheme. So yes, you still need to understand the difference between "das" and "dass", in the same way you need to understand the different between "there" and "they're" in english.