Butterworth-like filter with no zero at Nyquist?
Is the attenuation caused by aliasing or something different, and what is it called?

The two standard methods of mapping an analog filter to a digital one are either the bilinear transform or the impulse invariant transform. Neither does a particularly good job at preserving the transfer function at higher frequencies. The bilinear transform will map whatever happens at &quot;infinite&quot; analog frequency to the digital Nyquist frequency. For any analog lowpass filter (Butterworth or not), this will indeed create the steep roll off at Nyquist.

Are there digital alternatives to the Butterworth or bilinear transform that do not suffer from the attenuation near Nyquist, or is this a usual feature?

There are plenty of alternatives. Impulse invariant is one of them, although it to tends overshoot by 6dB at Nyquist. One heuristic method is to start with bilinear, and then move the zeros from z = -1 towards z = 0 until you have something you &quot;like&quot; by whatever criteria is important for your application.

What filter could an old resource-strained synthesizer have used to achieve a response without the attenuation?

A biquad is a biquad is a biqaud. Computational cost is not a function of the specific filter coefficients. There is really no difference between a Butterworth or any other second order filter.