As has been said before, mappability to the 'human genome' depends on a number of factors, among these the reference version and type of reads, for which you are interested in GRCh38 and 2x150bp reads. Although I am not aware of numbers accounting for these particular reference and type of Illumina reads, the 1000 genomes project has provided the community with a similar and close estimation that you might be interested in considering regarding your inquiry.
Similar to your question, the 1K project estimated 'the proportion of the human genome that is less accessible to short reads'. In these estimates the human genome is GRCh37 and the types of reads in question are mostly 2x Illumina with a mixture of lengths with the longest being up to 250bp. In these estimates each base in the human genome is considered (and marked) 'less accesible' according to these criteria:
L - depth of coverage is much lower than average
H - depth of coverage is much higher than average
Z - too many reads with zero mapping quality overlap this position
Q - the average mapping quality at the position is too low
Each of these criteria has "standard" and "strict" thresholds for a base to be considered - or not - in each category. You can read more in the link below:
According to the strict thresholds, the human genome has about 16.8% of "Z" and 3.1% of "Q" bases, respectively. Considering the "Z" and "Q" criteria as a proxy for ~ mapq=0, about 19.9% of the human genome can not be uniquely mapped.