Confusingly, on my machine, both the current git version and 2.28.0 give noun forms:

rupert@skate /s/n/maxima> ./maxima-local Maxima 5.28.0_130_g925e3f2_dirty http://maxima.sourceforge.net using Lisp SBCL 1.0.57.0.debian Distributed under the GNU Public License. See the file COPYING. Dedicated to the memory of William Schelter. The function bug_report() provides bug reporting information. (%i1) integrate(abs(sin(x)),x,0,2*%pi); 2 %pi / [ (%o1) I abs(sin(x)) dx ] / 0 (%i2) integrate(abs(cos(x)),x,0,2*%pi); 2 %pi / [ (%o2) I abs(cos(x)) dx ] / 0 (%i3) rupert@skate /s/n/maxima> maxima Maxima 5.28.0 http://maxima.sourceforge.net using Lisp GNU Common Lisp (GCL) GCL 2.6.7 (a.k.a. GCL) Distributed under the GNU Public License. See the file COPYING. Dedicated to the memory of William Schelter. The function bug_report() provides bug reporting information. (%i1) integrate(abs(cos(x)),x,0,2*%pi); 2 %pi / [ (%o1) I abs(cos(x)) dx ] / 0 (%i2) integrate(abs(sin(x)),x,0,2*%pi); 2 %pi / [ (%o2) I abs(sin(x)) dx ] / 0